JP2010043312A - Steel sheet for porcelain enameling having fishscale resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet for porcelain enameling which has improved fish-scale resistance and can secure workability equal to that of the conventional material. <P>SOLUTION: Disclosed is a steel sheet for porcelain enameling which has excellent fish-scale resistance having a composition comprising 0.0005 to 0.0030% C, ≤0.05% Si, 0.01 to 0.2% Mn, ≤0.02% P, ≤0.006% S, ≤0.005% Al, 0.001 to 0.003% N and 0.015 to 0.05% O, acid insoluble Nb: ≥0.025% and acid soluble Nb: 93/12×(%C)+93/14×(%N)+0.040 or above, also satisfying 93/12×(%C)+93/14×(%N)+0.040 or below, and the balance Fe with inevitable impurities, and inclusions with the major axis of ≥1 μm having a crystal structure of MnNb<SB>2</SB>O<SB>6</SB>are present in the steel sheet. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a steel plate for enamel having excellent nail skipping resistance.

  An enamel is a material in which a glass component is thinly baked on the surface of a metal plate, and combines the strength of the metal with the corrosion resistance and aesthetics of the glass. Because the surface is hard and smooth, it is not sanitized and is hygienic. For this reason, it is used in a wide range of fields such as kitchenware, medical instruments, and scientific containers. An enamel steel plate is manufactured by applying a glaze to a steel plate after forming and firing it in a high-temperature atmosphere at 800 ° C. or higher. For this reason, defects specific to enamel such as skipping nails and black spots may occur.

  Nail skipping is a process in which hydrogen contained in the glaze diffuses and penetrates into the steel during firing and is released as hydrogen gas from the steel during cooling and accumulates at the interface between the enamel layer and the steel plate, causing high pressure to repel the enamel layer. It is a defect of the enamel layer caused by flying. This is called nail skipping because the shape of the repelled trace resembles the shape of a nail.

  Recently, demands for steel sheets from enamel product manufacturers have become stricter, and further improvements in anti-jaw resistance and improved formability that allow more complex forming are required.

  In order to prevent claw skipping, it is important to provide a site for trapping hydrogen inside the steel plate to suppress rapid hydrogen accumulation at the steel plate / enamel layer interface. Therefore, a steel plate for enamel has been proposed in which a large amount of inclusions (oxides) and precipitates are generated and dispersed to improve the hydrogen trapping ability.

JP 2001-316760 A JP-A-10-183300 JP 2005-510624 A

Patent Document 1 forms inclusions mainly composed of MnO. Voids generated around inclusions by rolling such as cold rolling are used as hydrogen trap sites. In addition to MnO-based inclusions, SiO ₂ -based, Al ₂ O ₃ -based, MnS-based, etc. can be seen as inclusions, but Al ₂ O ₃ -based materials have been found to contribute to improved anti-nail resistance. However, the MnO system and the MnS system have a hydrogen trap effect. In Patent Document 1, a large number of dislocations existing at the crystal grain boundaries are also used as hydrogen trap sites. Therefore, in order to suppress the coarsening of crystal grains, the composition, size, shape, ratio, number of inclusions are suppressed. Is stipulated.

  In Patent Document 2, Nb oxide, Nb carbonitride, B oxide, and B nitride are generated to improve the resistance to claw skipping. The point of utilizing trapping hydrogen in the gaps between inclusions and precipitates and the ground iron is the same as utilizing the void generated around the inclusions in Patent Document 1. In order to suppress problems when the content of oxygen that forms inclusions (oxides) is excessive, such as pinholes during continuous casting, bulky inclusions, and blistering defects during pickling during pretreatment of enamel Further, the content of oxygen is made lower than that of the conventional steel, whereby the nail fly resistance, which is lower than that of the conventional material, is compensated by the generation of Nb and B carbonitrides and the annealing conditions. Unlike Patent Document 1, Mn is said to have a small effect on enamelability including anti-nail resistance.

  Patent Document 3 uses an oxide as in Patent Documents 1 and 2, but is formed by crushing the oxide in the cold rolling process rather than using the gap between the oxide and the ground iron. Emphasizes the use of gaps inside the oxide. As the composition of inclusions, Nb—V—Mn—Si—Fe composite oxide is mentioned, but the composition is not particularly defined. Rather, it is described that, for example, the oxide composition changes greatly due to the change in the amount of Mn, and consequently has a great influence on the state of stretching and crushing of the oxide during rolling, and the change in the hydrogen trapping ability due to this is described. In order to prevent this, it is a feature of Patent Document 3 to control the amount of density change when the steel plate is held at a high temperature for a long time. Specifically, the density change before and after annealing for 20 hours at a temperature of 850 ° C. in hydrogen gas needs to be 0.02% or more. Density change is interpreted as an index representing the activity of hydrogen trapping ability of the inner wall of the void in steel.

  In this way, various types of hydrogen trap sites such as inclusions and precipitates are used, but depending on the type of enamel glaze and firing conditions, nail skipping may occur and the user's demands continue to be severe Is not necessarily enough. And when the density change of patent document 3 is used as a parameter | index, a long-time evaluation test is required, and it causes the increase in a process, the delivery date, and the increase in manufacturing cost. The present invention has been devised to solve the above-described problems, and it is intended to provide a steel plate for an enamel that can improve the nail fly resistance and can ensure the workability equivalent to that of a conventional material. Objective.

For this purpose, the gist of the present invention is as follows. In addition, unless otherwise indicated, the% display in element content described in this specification shall be mass%.
(1) In mass%,
C: 0.0005 to 0.0030%,
Si: 0.05% or less,
Mn: 0.01 to 0.2%,
P: 0.02% or less,
S: 0.006% or less,
Al: 0.005% or less,
N: 0.001 to 0.003% or less,
O: 0.015-0.05%
Acid insoluble Nb: 0.025% or more Acid soluble Nb: 93/12 × (% C) + 93/14 × (% N) or more and 93/12 × (% C) + 93/14 × (% N) +0.040 or less,
The balance is a steel plate made of Fe and inevitable impurities,
A steel plate for enamel having excellent nail flying resistance, characterized in that inclusions having a major axis of 1 μm or more having a crystal structure of MnNb ₂ O ₆ are present in this steel plate.
(2) The mass ratio of total-Nb and Mn, which is the sum of acid-insoluble Nb and solid-soluble Nb, satisfies (% Mn) / (% total-Nb) ≦ 1.2 (1) ) Steel plate for enamel described in the above.
(3) The average composition of inclusions having the crystal structure of MnNb ₂ O ₆ satisfies (% MnO) + (% Nb ₂ O ₅ ) ≧ 85, as described in (2) above Steel plate for enamel.

  According to the present invention, it is possible to produce a steel plate for enamel, which greatly improves the resistance to catching nail, greatly reduces defects in the enamel layer, or has no defects. Processability can be secured at the same level as conventional materials.

  Next, the operation brought about by the constituent elements of the present invention, that is, the technical idea of the present invention will be described in detail. The embodiment of the present invention will be described later together with the reason for limiting the component range.

  First, the technical idea of the present invention will be described. The present inventors consider that it is effective to further increase the voids formed around and inside the inclusions during rolling, in order to further improve the nail flying resistance and the hydrogen trapping ability. We focused on the effect of type and composition. As will be described later, a number of steel plate samples with different inclusion types and compositions were prepared, and nail fly resistance, hydrogen trapping ability, and void formation behavior around and inside the inclusions were examined.

In the experiment, the hydrogen trap ability was evaluated by an electrochemical hydrogen permeation test method. This test method is a method for measuring the time for hydrogen to permeate in the thickness direction of the steel sheet, and is generally used for evaluating the hydrogen trapping ability of a steel sheet for enamel. In this test method, the longer the measured time, the higher the hydrogen trapping ability. The evaluation result of this test method is indicated by a T value (min / mm ² ) obtained by dividing the hydrogen permeation time (min) by the square of the plate thickness (mm). It was considered that the value should be at most 15 (min / mm ² ). However, in the present situation where user requirements are becoming strict, a steel plate having a higher T value is desired. In the experiment, in order to investigate the relationship between the T value and the type and composition of inclusions, the shape of inclusions in the cross section of the steel sheet and the surroundings of the inclusions (ie, the interface between inclusions / ground iron) ) And the formation of voids inside the inclusions.

As a result, in order to improve the hydrogen trapping capability (increase the T value), MnNb ₂ O ₆ in which MnO and Nb ₂ O ₅ are combined at a molar ratio of 1: 1, that is, an inclusion mainly composed of columbite. It has been found that it is particularly effective to disperse (hereinafter, this inclusion may be referred to as MnNb ₂ O ₆ -based inclusion). In Patent Documents 1 to 3, there is no description about MnNb ₂ O ₆ (Colombite).

The following points can be considered as the reason why MnNb ₂ O ₆ -based inclusions are more effective than conventionally used inclusions such as MnO. That is, (a) since it is difficult to deform during rolling, it is easy to form voids at the interface with the steel, (b) cracks inside inclusions during rolling, and it is easy to form new voids, (c ) Since many MnNb ₂ O ₆ inclusions are finely dispersed in the slab after solidification, and eventually in the steel plate, the interfacial area between the inclusions / base metal increases, so the formation of voids by the mechanism (a) is promoted. It is three points of being done. The combined effect of these increases the number of voids formed around the inclusions (inclusion / base metal interface) that are hydrogen traps and inside the inclusions, so that the hydrogen trap ability, that is, the T value is increased. It is thought to be higher.

As for (a), since MnNb ₂ O ₆ inclusions are less deformed during rolling than MnO inclusions conventionally used, voids are likely to be formed at the interface with the base iron. As a result of observation, void formation was observed around inclusions having a major axis of 1 μm or more.

About (b) is mainly observed in major diameter MnNb ₂ O ₆ inclusions of more than 5 [mu] m. If the major axis is 5 μm or more, cracks are likely to occur inside the inclusions during cold rolling, and the fracture surface of the crack part deviates in the rolling direction to form new voids, which become hydrogen trap sites. It is done.

  Regarding (c), it is involved that both Mn and Nb are weak deoxidizing elements. The number of inclusions remaining in the slab of oxides of weak deoxidation elements such as Mn and Nb is larger than that of strong deoxidation elements. This is because the oxide of the weak deoxidizing element is difficult to be coarsened in the molten steel and continues to be suspended in the molten steel while being fine, and the floating speed to the molten metal surface is low. On the other hand, when a strong deoxidizing element such as Al or Ti is added, the generated oxide grows rapidly and becomes coarse, so the flying speed is high and the surface easily floats. As a result, the inclusions in the molten steel remaining by the time of casting are less than in the case of weak deoxidizing elements.

In the case of deoxidizing with a weak deoxidizing element, it is also advantageous that the dissolved oxygen in the molten steel after deoxidation is high. Since a large amount of dissolved oxygen remains in the molten steel, a large amount of oxygen segregates together with Mn and Nb between the solidified structures during solidification. As a result, a large number of fine oxides are generated between the solidified structures. Since such an oxide is generated within a limited time from the start of solidification to the end of solidification, it hardly becomes coarse and does not float because it is formed between solidified structures. As a result, a large number of oxides are dispersed between the solidified structures while being fine. Furthermore, since the interfacial energy between inclusions / molten steel decreases when the dissolved oxygen concentration after deoxidation is high, it becomes more difficult to agglomerate and coalesce in the molten steel, which advantageously affects the fine dispersion of inclusions. On the other hand, in the case of a strong deoxidizing element, the dissolved oxygen in the molten steel after deoxidation is very low, so that almost no oxide is generated between the solidified structures during solidification. For these reasons, it is considered that many MnNb ₂ O ₆ inclusions are finely dispersed in the molten steel, in the slab after solidification, and in the steel plate. In this way, as a result of the increase in the interfacial area between inclusions / base metal, the void volume formed by the mechanism (a) increases.

For the reasons described above (a) to (c), forming inclusions containing MnNb ₂ O ₆ in the steel sheet promotes void formation around and inside the inclusions, which are hydrogen trap sites. Therefore, it is effective for improving the hydrogen trapping ability, that is, improving the nail flying resistance. Therefore, by making inclusions having a major axis of 1 μm or more present in the steel sheet have a crystal structure of MnNb ₂ O ₆ , it is possible to improve the resistance to catching nails and increase the T value from 15 than before. I found out that I can.
Incidentally, among inclusions having a major axis of 1 μm or more, inclusions containing 50% or more by mass ratio of MnNb ₂ O ₆ tend to be stable to a T value of 17 or more when the number ratio is 50% or more. This is preferable because variations due to the portion of the steel sheet are reduced.

And, as a result of studying measures for stably forming MnNb ₂ O ₆ -based inclusions, total-Nb (hereinafter referred to as the total of Mn, acid-insoluble Nb, and acid-soluble Nb contained in the steel sheet) It was found that the concentration ratio (denoted as t-Nb) has an effect. When the ratio of the amount of Mn to the amount of t-Nb (% Mn) / (% t-Nb) is changed, the ratio of the amount of MnO to be produced and Nb ₂ O ₅ is changed. Incidentally, when (% Mn) / (% t-Nb) is high, the amount of MnO produced is larger than that of Nb ₂ O ₅ , so that an MnNb ₂ O ₆ crystal structure and excess MnO are produced. When there is much this excess MnO, the hydrogen trap ability of a steel plate will fall. This is presumably because MnO is more easily stretched during rolling than MnNb ₂ O ₆ , voids are less likely to be formed at the interface with the base iron, and cracks are less likely to occur inside. For this reason, the smaller the excess MnO, the better the hydrogen trapping ability. Specifically, it is desirable that (% Mn) / (% t−Nb) ≦ 1.2 to ensure a T value of 21 or more. Became clear in experimental findings.

Furthermore, as a result of a detailed investigation of the relationship between the composition of inclusions in the steel sheet and the hydrogen trapping capability, reducing the content of compounds other than MnNb ₂ O ₆ contained in the inclusions improves the hydrogen trapping capability. Found to be effective to do. In the inventor's experiment, even when (% Mn) / (% t-Nb) is a close value, the T value may be different.

This is because the deformability of inclusions during rolling changes depending on the content of compounds other than MnNb ₂ O ₆ contained in the inclusions. As a result, the ability to form voids around and inside the inclusions changes. I found out that it was. For example, if the content of SiO ₂ or MnS other than MnNb ₂ O ₆ contained in the inclusions is increased, the inclusions are easily deformed during rolling, and voids around and inside the inclusions are less likely to be formed. This is not preferable because the T value decreases. Specifically, the average composition of MnNb ₂ O ₆ inclusions having a major axis of 1 μm or more and (% MnO) + (% Nb ₂ O ₅ ) of 85 or more is preferable because the T value can be 24 or more. From the experimental findings, it became clear.

  Here, the reason why the inclusions to be evaluated are those having a major axis of 1 μm or more is that the size of the voids around and inside the inclusions can be observed with an electron microscope. With inclusions smaller than this, it is difficult to determine the formation of voids. Based on the above three technical ideas, the requirements of the present invention are configured.

Next, in the embodiment of the present invention, first, the reason for limiting the component range will be described in detail.
C: 0.0005 to 0.0030%
C is an element necessary for ensuring the strength of the steel sheet. The upper limit of the C content is 0.0030%. This is because if it is higher than this, the ductility necessary for press working cannot be obtained. On the other hand, the lower limit is set to 0.0005%, which is a concentration at which decarburization can be performed within a practical operation time.

Si: 0.05% or less Si is an element useful for deoxidation or strength improvement, but if the oxide is contained in a large amount in MnNb ₂ O ₆ inclusions, the inclusions have a low melting point. As a result, since it becomes easy to extend | stretch at the time of rolling, the space | gap formation capability of an inclusion falls. Moreover, when Si concentration becomes high, a steel plate will become hard and workability will fall. For these reasons, the upper limit of the Si content is 0.05%. The lower limit is not particularly defined and includes 0%.

Mn: 0.01 to 0.2%
Mn is an essential component in the present invention in order to form MnNb ₂ O ₆ and must contain an appropriate amount. In order to react with Nb in the concentration range described later to form MnNb ₂ O ₆ , the lower limit is set to 0.01% as the minimum necessary concentration. On the other hand, even if added over 0.2%, the hydrogen trapping ability is not improved and it is saturated. This is presumably because coarse MnNb ₂ O ₆ is easily generated. In addition, if Mn is contained excessively, the hardness of the steel sheet increases and the workability decreases. For the above reason, the upper limit is made 0.2%.

P: 0.02% or less P has an effect of increasing the strength by solid solution, but if contained excessively, it is not preferable because the ductility of the steel sheet is deteriorated. Therefore, in the present invention, the upper limit of the P content is 0.02%. The lower limit includes 0%.

S: 0.006% or less S forms MnS having a low melting point and extends during hot rolling, so that the ductility of the steel sheet is deteriorated as the content increases. In addition, when a large amount of MnS is generated around the MnNb ₂ O ₆ inclusions, voids as hydrogen trap sites that should originally be formed at the interface between the inclusions and the ground iron are filled during rolling. Furthermore, when a large amount of S fixes Mn as MnS, the amount of Mn necessary to form MnNb ₂ O ₆ is reduced, resulting in a reduction in the amount of MnNb ₂ O ₆ produced. For this reason, the allowable upper limit of the S content is set to 0.006%. The lower limit includes 0%.

Al: 0.005% or less Al is a strong deoxidizing element. When added in a large amount, Al ₂ O ₃ is preferentially produced and oxygen in the molten steel is consumed. As a result, MnNb ₂ O ₆ is sufficient in the molten steel. Even when MnNb ₂ O ₆ -based inclusions are formed, the inclusions become coarse and easily float when the Al ₂ O ₃ concentration in the inclusions increases. For this reason, the number of inclusions remaining in the molten steel until the start of solidification is remarkably reduced. Furthermore, since the dissolved oxygen concentration after addition of Al is low, the amount of inclusions produced during solidification is also reduced. In this way, when the Al content is excessive, the MnNb ₂ O ₆ inclusions are not sufficiently formed and the hydrogen trapping capability is greatly reduced. Therefore, the allowable upper limit of the Al content is set to 0.005%. . The amount of Al in this case is the total-Al amount that is the sum of acid-soluble Al and acid-insoluble Al. The lower limit is not particularly defined and includes 0%. In addition, in the present invention, if the Al addition amount is adjusted according to the dissolved oxygen concentration in the molten steel before Al addition, the slab and thus the Al content of the steel sheet can be 0.005% or less. It is possible to use Al as a preliminary deoxidizer.

N: 0.001 to 0.003%
N acts to improve the strength, but the ductility decreases as the content increases, so the upper limit is made 0.003%. On the other hand, in actual operation, it is difficult to denitrify to less than 0.001%, so 0.001% is made the lower limit.

O: 0.015-0.05%
O is an index of the amount of inclusion indicating that the higher the content, the more the amount of inclusion. In order to improve the nail flying resistance more than the present state, 0.015% or more is necessary, so the lower limit is made 0.015%. On the other hand, if the O content exceeds 0.05%, coarse inclusions cause a decrease in ductility, so the upper limit is made 0.05%.

Acid insoluble Nb: 0.025% or more Nb contained in the steel sheet can be divided into acid insoluble Nb and acid soluble Nb. Both can be separated and analyzed using the property that Nb oxide is insoluble in acid (except for hydrofluoric acid and sulfuric acid). Of these, acid-insoluble Nb is the most important element contributing to the expression of the hydrogen trapping ability in the present invention, and is mainly present as MnNb ₂ O ₆ in the steel sheet. According to the experiment results of the inventors, when the acid-insoluble Nb is less than 0.025%, it cannot be increased beyond the conventional T value of 15. Then, even if X-ray was irradiated to the inclusions electrolytically extracted from the steel sheet was investigated a crystal structure in X-ray diffraction method, the diffraction pattern of MnNb ₂ O ₆ can not be clearly detected, the generation of MnNb ₂ O ₆ It is difficult to verify. Therefore, the content of acid-insoluble Nb is set to 0.025% or more. The more the acid-insoluble Nb is, the better in terms of improving the hydrogen trapping ability. However, if it exceeds 0.2%, the inclusions become coarse and the workability decreases, so that the content is preferably 0.2% or less.

Acid-soluble Nb: 93/12 × (% C) + 93/14 × (% N) or more and 93/12 × (% C) + 93/14 × (% N) +0.040 or less The acid-soluble Nb that is not formed is divided into solid solution Nb that is solid-solved in the steel sheet and Nb that forms carbonitride. In order to ensure good workability and non-aging properties, it is necessary to fix C and N. For this reason, the amount of acid-soluble Nb is 93/12 × (% C) + 93/14 × (% N) or more. On the other hand, excessive Nb beyond fixing C and N increases the recrystallization temperature. When heated at the same temperature, the recrystallization rate decreases, resulting in a decrease in ductility. For this reason, the allowable upper limit of excess Nb was made 0.040%. Therefore, the upper limit of the acid-soluble Nb that combines Nb and excess Nb necessary for C and N fixation is 93/12 × (% C) + 93/14 × (% N) +0.040 or less.

  Although it is desirable to reduce other inevitable impurities as much as possible, it is acceptable as long as it does not adversely affect the nail flying resistance and ductility of the steel sheet according to the present invention.

  In order to produce the steel sheet of the present invention, the molten steel in the above component range is melted using, for example, a converter and a vacuum degassing apparatus, continuously cast into a slab, and heated by a general ordinary method. , Hot rolling, cold rolling, annealing. Since the deformation and crushing behavior of inclusions during rolling does not change drastically as long as it is within the condition range for creating a normal enamel steel plate, it is possible to obtain an enamel steel plate with excellent hydrogen permeability according to the present invention. it can.

Inclusions having a crystal structure of MnNb ₂ O _6, i.e., in order to verify that it contains MnNb ₂ O ₆ may be, for example, using X-ray diffraction method. In this method, the steel sheet is electrolyzed, the inclusions are electrolytically extracted, and the diffraction pattern is measured by the X-ray diffraction method. In actual inclusions, but often it contains compounds other than MnNb ₂ O _6, compared to the diffraction pattern obtained from a standard sample of MnNb ₂ O _6, research it contains a diffraction pattern of MnNb ₂ O ₆ Just do it.
Although it is difficult to quantitatively evaluate the MnNb ₂ O ₆ content from the diffraction pattern alone, it is preferable that the diffraction line peak of MnNb ₂ O ₆ is higher and clearer in the diffraction pattern obtained from the sample.
The inclusion composition can be quantitatively analyzed by using an EPMA apparatus installed in an electron microscope.

  Hereinafter, the present invention will be described in detail based on examples and comparative examples.

  It shows about the manufacturing method of the steel plate of the component shown in Table 1. First, 300 t of molten steel decarburized and refined in a converter was decarburized to 0.003% or less using an RH vacuum degasser. Next, Al was added for preliminary deoxidation to adjust components other than Nb, and finally ferroniobium was added to adjust the Nb concentration. This molten steel was cast by a continuous casting machine to obtain a cast piece having a width of 1200 mm and a thickness of 250 mm, and gas cutting to obtain one coil unit. From the slab thus obtained, a 1.2 mm thick steel plate was prepared through the following steps. That is, the slab was heated to 1130 to 1150 ° C., then hot-rolled to 4.0 mm at a finishing temperature of 925 to 975 ° C., and wound at 730 to 770 ° C. Next, after cold-rolling to 1.2 mm (reduction rate 70%), continuous annealing was performed at 730 to 770 ° C., and temper rolling was performed at a reduction rate of 1.0%.

Samples were collected from the steel plates thus prepared, and the hydrogen trap ability, the presence or absence of MnNb ₂ O ₆ in the inclusions, the inclusion composition, and the workability were evaluated. For the hydrogen trapping ability, the T value was measured by an electrochemical hydrogen permeation test method. T-value measurement samples were collected from three locations in the width direction of the steel plate, and the average value was obtained. The evaluation standard was determined to be acceptable when the average of T values at three locations was 17 or more. A higher T value is preferable. The presence or absence of MnNb ₂ O ₆ in the inclusion was examined by X-ray diffraction method for the crystal structure of the electrolytic residue collected by electrolyzing the steel sheet. The case where the diffraction pattern of MnNb ₂ O ₆ was detected was regarded as acceptable. The inclusion composition was analyzed by an EPMA apparatus attached to the electron microscope after observing the inclusion with a scanning electron microscope after polishing the steel plate surface. The observation target was 20 inclusions having a major axis of 1 μm or more, and the average composition was calculated by the method described below. Incidentally, the 20 inclusions are based on the fact that the present inventor has already found that sufficient reliability can be obtained.
Mn, Nb, Al, Si, and O are mainly detected as oxide constituent elements, but other components that are considered to be mixed in part from refractory materials may be detected. When S was detected, it was considered as MnS. Quantitative analysis results of the above elements were converted to MnO, Nb ₂ O ₅ , Al ₂ O ₃ , SiO _{2 and the} like as oxide compositions, and oxide inclusion compositions were calculated by removing MnS. For workability, a tensile test was performed using a JIS No. 5 test piece. As the evaluation criteria, an extension amount of 50% or more was regarded as acceptable. Finally, the steel sheet was coated with glaze and fired at 850 ° C. to evaluate the occurrence of nail skipping.

Table 1 shows the component values of the steel sheet. Sample No. 1 to 10 are examples of the present invention. 11 to 13 are comparative examples. In the comparative example, MnNb ₂ O ₆ was not detected from the inclusions. The average T value was 15 or less, and the hydrogen permeability was the same as before. In the enamel layer after firing, two or more nail jumps having a length of 2 mm or more occurred per 1 m of the steel plate.

On the other hand, no. In 1 to 10, MnNb ₂ O ₆ was detected, the average value of T values was 17 or more, and the hydrogen permeability was good. No. In the steel sheets of Nos. 2 and 3, the T value of each part was stable within an average value of ± 1. 1, the average value was within the range of ± 1.5. No. In the steel plate No. 1, the number ratio of inclusions containing 50% or more of MnNb ₂ O ₆ by mass ratio remained at 45%. 2 and 3 were 50% and 55%, respectively. The elongation value was 50% or more, and the workability was also acceptable. The nail skipping property of the enamel layer after firing is also good. In Nos. 1 to 3, only one nail jump of 1 mm or less occurred per 1 m of the steel plate.

Among the examples of the present invention, no. 3 to 10, since (% Mn) / (% t-Nb) is 1.2 or less, voids are likely to be formed around and inside the inclusions, and the average T value of each sample is 21 or more. It was good. There is little variation in T value depending on the part. In all the samples from 3 to 10, the T value of each part was stable within an average value ± 1. The number ratio of inclusions containing 50% or more of MnNb ₂ O ₆ by mass ratio was 50% to 100%. The nail skipping property of the enamel layer after firing was even better, and only one spot-like defect occurred per 1 m of the steel plate.

Furthermore, No. of this invention example. 7 to 10 were the average composition of inclusions, and (% MnO) + (% Nb ₂ O ₅ ) was 85% or more, and the amount of oxides other than MnO and Nb ₂ O ₅ was small. For this reason, one layer of voids was formed around and inside the inclusions, and the average T value was very good at 24 or more. No defects occurred in the enamel layer after firing.

Claims (3)

% By mass
C: 0.0005 to 0.0030%,
Si: 0.05% or less,
Mn: 0.01 to 0.2%,
P: 0.02% or less,
S: 0.006% or less,
Al: 0.005% or less,
N: 0.001 to 0.003%,
O: 0.015-0.05%
Acid insoluble Nb: 0.025% or more Acid soluble Nb: 93/12 × (% C) + 93/14 × (% N) or more and 93/12 × (% C) + 93/14 × (% N) +0.040 or less,
The balance is a steel plate made of Fe and inevitable impurities,
A steel plate for enamel having excellent nail flying resistance, characterized in that inclusions having a major axis of 1 μm or more having a crystal structure of MnNb ₂ O ₆ are present in this steel plate.   The mass ratio of total-Nb and Mn, which is the sum of acid-insoluble Nb and solid-soluble Nb, satisfies (% Mn) / (% total-Nb) ≦ 1.2. Steel plate for enamel. The steel sheet for enamel according to claim 2, wherein an average composition of inclusions having a crystal structure of MnNb ₂ O ₆ satisfies (% MnO) + (% Nb ₂ O ₅ ) ≧ 85.